The invention relates to rollers, such as developer or charge rollers, used in laser printers.
A laser printer often includes a digital light emitter photoconductor drum, a charge roller, a developer roller, a developer blade, a transfer roller, and a toner storage unit. During printing, the transfer roller supplies toner to the developer roller, and the developer blade forms the toner into a thin, even layer on the surface of the developer roller. The charge roller charges the photoconductive drum with a positive or negative charge. After the photoconductive drum has been exposed a light emitter, the surface of the photoconductor drum forms an electrostatic latent image, and the developer roller transfers toner to the portion of the drum surface to form the toner image. The laser printer may also include a toner-removal wiper that removes excess toner from the surface of the developer roller after the developer roller has contacted the photoconductor drum. The toner on the drum subsequently is transferred to paper, and then fuses to form the print.
Many different designs of laser printers are known. They include Shiraki et al., U.S. Pat. No. 5,768,668; Sato, U.S. Pat. No. 5,752,146; Park, U.S. Pat. No. 5,727,022; Okada et al., U.S. Pat. Nos. 5,669,047 and 5,655,197; Sakaguchi, U.S. Pat. No. 5,602,631; Iguchi et al., U.S. Pat. No. 5,600,417; Ikeda et al., U.S. Pat. No. 5,367,367; Kogo et al., U.S. Pat. Nos. 5,324,885 and 5,214,239; Kinoshita et al., U.S. Pat. Nos. 5,311,264 and 5,287,150; Nishio, U.S. Pat. Nos. 5,241,343, 5,076,201, and 5,062,385; and Goyert et al., U.S. Pat. No. 4,521,582; all of which are incorporated herein by reference.
The developer roller is cylindrical and typically includes a central shaft surrounded by a synthetic rubber or urethane elastomer portion. Often, high resolution laser printers include a developer roller having a resistivity of 1-3E9 ohm-cm and a hardness of 53-60 Shore A on roller. High speed ( greater than 15 pph), high resolution (1200 dpi) laser printers often use relatively small (1-8 um) toner particles in which the toner has a relatively low melting point. In these printers the developer roller sometimes melts a portion of the toner prior to transfer to the photosensitive drum. This can make the toner sticky, and lead to a ghosting problem in print quality. In addition, sometimes the developer roller does not pick up sufficient toner, resulting in a light print.
In general, the invention features a roller (e.g., a developer roller or charge roller) for use in laser printers. The roller generally includes an inner shaft surrounded by an outer, solid thermoset urethane portion. By solid, it is meant that the thermoset urethane portion is not a foam.
The thermoset urethane has a hardness of 30-50 Shore A, preferably 35-45 Shore A, on cube (flat surface), or 40-50 Shore A on roller (curved surface). On cube means that a cube of the urethane is tested for hardness. On roller means the roller itself is tested for hardness. Generally, the hardness of the thermoset urethane is 3-5 A higher on roller than on cube, due to taking the measurement on the curve surface of the roller. Developer rollers made using thermoset urethane having this hardness are less apt to cause melting of toner. As a result, they provide good print quality, with limited if any ghosting. In addition, the developer roller has good conformability with the toner blade and as a result receives a more uniform thickness of toner.
The thermoset urethane also has a volume resistivity of 1E6 ohm-cm to 9E8 ohm-cm, and preferably 3E6 ohm-cm to 8E8 ohm-cm. The resistivity, and conductivity, of the roller is uniform, which enhances the print quality. The roller has uniform resistivity and conductivity in part because the thermoset urethane does not include any plasticizers or other liquids that migrate to the surface of the roller. The roller also has uniform resistivity and conductivity in part because the metallic salt in the urethane used to provide the conductivity is completely dissolved (i.e., is a solid solution) and complexed in the thermoset urethane. Finally, the thermoset urethane has excellent reversion or hydrolysis resistance and preferably exhibits a stable (i.e., changes no more than 3xc3x97101) volume resistivity even under a change of humidity of 10% to 90% and a change of temperature of 10xc2x0 C. to 40xc2x0 C.
The invention also features methods of preparing the roller including the thermoset urethane, as well as laser printers including the roller.
Other features and advantages will be apparent from the description of the preferred embodiments thereof, and from the claims.